1. Field of the Invention
The invention involves radiation treatment of polymeric materials, including treatment of polytetrafluoroethylene (xe2x80x9cPTFExe2x80x9d) to degrade the material into lower molecular weight forms. That is the electron beam degrades the molecular chain of the polymer thereby reducing the molecular weight. More specifically, the invention has to do with the use a vibratory table to move polymer, such as PTFE, in an even layer, that is at a consistent thickness or depth under an electron beam. The vibratory table apparatus and methods in accordance with the present invention accomplish several desired improvements over apparatus and methods known in the art, and resolves certain deficiencies associated with radiation treatment, particularly radiation treatment of fluoropolymers such as PTFE. The apparatus and methods provide an even layer of material on a continuous basis under an electron beam which allows for more efficient processing without radiation losses and labor requirements of other processes, provides for an enclosed environment which protects against gas evolution and provides for continuous cooling. These and other advantages over radiation process known in the art are achieved with the apparatus and methods of the invention as described herein.
The polymeric material is treated in an enclosed environment which permits treatment to be performed in dedicated environments, including an inert environment and environments rich in particular gases, such as oxygen and nitrogen, or combinations of gases. This feature of the invention allows the apparatus and process to be adapted to permit reaction between materials on the table and chemicals permitted to enter the environment where the radiation treatment occurs. The method can also be adapted to combine at least two materials on the vibratory table to allow for a reaction or reactive coating, such as copolymerization, between the materials using the energy of the electron beam. The apparatus comprises a dual loop cooling water system which provides for the efficient removal of the significant heat generated during the radiation processing, and other cooling systems dedicated to other pieces of apparatus used in the process. The material can be delivered to the vibratory table by any means capable of transporting polymer materials, including a pneumatic flow system, and any means capable of transporting polymer materials, including a pneumatic flow system, can be used to convey treated material from the area of the vibratory table. The radiation processing can embrittle the polymeric material due to molecular wright degradation thus facilitating the physical breaking of the treated polymers in the pneumatic flow system to allow for the reduction in material particle size. Use of a pneumatic flow system with the vibratory table provides for the reduction of the particle size of the polymeric material to about 30 to about 3,000 microns.
2. The Related Art
Radiation treatment has been employed to polymerize organic substances and for treatment of polymers. For example, U.S. Pat. No. 2,921,006 to Schmitz et al. describes polymerization of monomers with an electron beam which is exposed to the monomer in a vacuum chamber partially filled with a cooling medium. An embodiment of the invention in U.S. Pat. No. 2,921,006 involves the continuous treatment of monomer moved through the electron beam on a thin sheet of stainless steel, about 0.002 inches thick, with side flanges of resilient material to prevent material dropping off the sides of the sheet. This patent does not involve the degradation of polymer and does not address process concerns and parameters pertinent to polymeric degradation with an electron beam, and particularly degradation of PTFE.
U.S. Pat. No. 3,081,485 to Steigerwald involves treatment of thermoplastic strands which have softening points below their decomposition points. The strands are moved through an electron beam in a vacuum chamber to soften the thermoplastic. U.S. Pat. No. 5,856,675 to Ivanovich et al. concerns the movement of polymer, on a continuous basis using a conveyor system through an electron beam to cross-link the material.
Radiation can be used to degrade the molecular weight of polymers such as PTFE. For example, U.S. Pat. No. 3,766,031 to Dillon discusses radiation treatment of PTFE with an electron beam at between about 5 and 25 Mrads which renders the PTFE capable of being comminuted to microfineness. This comminuted PTFE is useful as a dry lubricant, for example, in paints and inks. The process employed in U.S. Pat. No. 3,766,031 is not efficient and not appropriate for modern processing because there is no method for cooling the PTFE and the PTFE is unevenly treated. U.S. Pat. No. 4,220,511 to Derbyshire discusses the degradation of unsintered PTFE to be ground into a powder with an average size less than 10 microns. A combination of a radiation at 50-150 Mrads and heat at 150xc2x0 F. to 600xc2x0 F. for approximately one half hour is used in the process to degrade the PTFE. U.S. Pat. No. 4,029,870 to Brown, et al. discusses dry lubricant PTFE obtained by subjecting the PTFE to xcex3-radiation at doses of between 2 and 20 Mrads and subsequently comminuting the PTFE to microfiness. U.S. Pat. No. 5,891,573 to Nueberg et al. addresses subjecting PTFE handled at a temperature below 66xc2x0 F. to a radiation source to obtain friable PTFE, and the PTFE may be combined with a wetting agent prior to irradiation.
Conventional methods of treating PTFE with electron beams, in use today, including those involving the use of trays are inefficient and results in PTFE products that are unevenly treated. The conventional methods result in uneven radiation treatment, and experiences other deficiencies, including the inability to control the processing environment, process gas evolution and the inability to effectively remove heat.
Several factors contribute to the inefficiency of tray irradiation of PTFE. The most significant occurs as a result of the beam penetration characteristics through the depth of material in the tray. Typically, the dose at the surface is taken as the nominal dose for the material. Beam energy and/or material depth is adjusted so that an equal dose is effected at the opposite surface of the material. Radiation which passes entirely through the product is not utilized. Radiation in excess of the nominal dose is likewise not used. This causes inefficiency, and in some instances may result in undesired properties of the resultant product. This depth-dose characteristic can cause processing inefficiency of up to 50%. Variations in material depth in the tray, gaps between the trays, and overscan of the tray, which is necessary to assure complete and uniform irradiation, can cause additional inefficiencies of approximately 5-15%.
The depth-dose characteristic, overscan and variations in material depth that occurs with tray processing all contribute to uneven treatment of the PTFE. Consequently, some of the PTFE may be overly treated and other portions of the PTFE may not receive adequate treatment. This results in inconsistent properties and, hence, unpredictable product quality of the processed PTFE.
Processing on trays can only be performed in batches requiring time and labor to place material on the tray and move trays into position and from the processing area after treatment. This requires that a human being be so integrally involved in the process that processing is generally performed in ambient conditions. Thus, the conventional tray process is limited in the inability to effectively control the environment where the processing occurs, and the process cannot be readily adapted to allow for reaction between PTFE and other materials, or to react or combine polymers in general.
Hydrogen fluoride gas generation from radiation treatment of fluoropolymers, such as PTFE, is a problem associated with the tray method and other known processes for degradation of fluoropolymers such as PTFE with an electron beam. Presence of hydrogen fluoride gas presents worker safety and environmental control issues. As discussed below, the hydrogen fluoride gas will interact with steam or condensed water resulting from cooling water methods which results in generation of hydrofluoric acid that not only causes additional worker safety issues and environmental control issues, but causes potential corrosion of equipment.
Radiation treatment of PTFE generates large amounts of heat and successful industrial processes for treatment of PTFE require removal of the thermal energy due to the electron beam from the processing area. The tray method is limited because, as in U.S. Pat. No. 3,766,031, there is no provision for heat transfer.
When methods of heat transfer such as direct cooling of the PTFE or water jackets are applied in conventional radiation processes, these methods are generally inefficient and have serious drawbacks. A conventional method for cooling during tray processing is applying water directly to the PTFE. As discussed above, the hydrogen fluoride gas generated from the electron beam treatment of the PTFE reacts with the cooling water to form caustic hydrofluoric acid. Besides the previously discussed health, safety, and corrosion issues caused by hydrofluoric acid, the direct cooling method generates an acidic waste stream which may require treatment or special considerations for disposal and other environmental controls. The hydrofluoric acid is caustic and this can corrode the apparatus and equipment used in the PTFE treatment process and gives rise to worker health and safety issues. Direct cooling of PTFE with water also inhibits the degradation process, preventing proper degradation of the PTFE and resulting in an inferior product.
Another method to remove heat during PTFE radiation treatment is the use of cooling water jackets. Use of conventional cooling water jackets to remove heat during PTFE radiation treatment generally require large volumes of water to effectively remove the vast amounts of heat generated in the process. Thus, cooling water jackets require large amounts of water resources which add to the costs and material requirements for PTFE processing. Also, during the cooling process with a water jacket, condensation can form during cooling which can mix with the hydrogen fluoride gas to form caustic hydrofluoric acid. As discussed above, hydrofluoric acid gives rise to wastes, such an acidic waste stream that may require special treatment or considerations for disposal and other environmental controls, corrosion of the apparatus and equipment and worker health and safety issues.
One method employed to inhibit hydrofluoric acid production, during the cooling process with a cooling jacket, is to heat the water to a temperature above the dew point of the gas in the surrounding environment to avoid condensation. This, however, reduces the heat transfer capabilities of the water thereby requiring larger amounts of water than cooling processes that do not require heated water. Furthermore, in order to elevate the temperature of the water, water to water heat exchangers may be necessary adding complexity to the process and equipment combined with additional capital and operating costs.
Efforts have been made to develop processes that provide for more even radiation treatment of PTFE to avoid the problems associated with the tray method and to avoid the traditional drawbacks associated with cooling systems. An apparatus and method for degrading PTFE by propelling the polymer in a fluidized bed of hot air into the path of an electron beam is disclosed in U.S. Pat. No. 5,296,113 to Luniewski. The apparatus has a means for generating an electron beam, a chamber where the PTFE interacts with the electron beam and a separator. Heated air at high pressure is injected into the chamber to move the PTFE around the chamber and through the path of the electron beam. The PTFE is said to be degraded to particles with sizes ranging from 10 to 400 microns which then can be use as a dry lubricant. The apparatus relies on direct air cooling of the PTFE during the process, and the apparatus is equipped with cooling water jackets for the bottom of the chamber and an aperture at the top of the chamber where the electron beam enters the chamber. The cooling water jackets are designed to cool the chamber, and not the PTFE, and the apparatus relies on rapid treatment of the PTFE so that the PTFE particles are not affected by the cooling water in the cooling water jackets.
U.S. Pat. No. 5,149,727 to Luniewski describes a method and apparatus for treating and degrading PTFE for use as a dry lubricant which utilizes simultaneous irradiation, grinding, agitation and air cooling. The apparatus has a means for generating an electron beam, and a grinding vessel. The grinding vessel includes air jets which can inject air at up to 200 pounds per square inch (xe2x80x9cpsixe2x80x9d) into the grinding vessel and paddles. The paddles cause the PTFE to move haphazardly into the path of the electron beam in the grinding vessel. The air injection cools the PTFE within the grinding vessel to a temperature below 620xc2x0 F., the melting point, and also assists in the movement of the particles within the vessel. The PTFE particles are degraded, primarily, by the radiation and grinding action that occurs within the vessel. The air injection alleviates the need for a cooling water system for the apparatus. A classifier attached to the apparatus causes the degraded PTFE particles to exit the grinding vessel when a desired particle size is achieved. The apparatus can treat PTFE on a batch basis in a typical time of four to nine hours and the process appears primarily developed for batch processing although the process is said to be adaptable for a continuous operation.
Movement of PTFE through an electron beam in a ribbon-blender type apparatus is discussed in U.S. Pat. Nos. 4,777,192 and 4,748,005 to Neuberg et al. The apparatus comprises a means for generating an electron beam and a chamber where PTFE is subjected to the electron beam. Material is delivered to the chamber by a screw conveyor. Stirrer paddles are located within the chamber which move the PTFE within the chamber. The chamber is also equipped with means for permitting flow of air into the chamber which tends to provide fluidizing of the PTFE. This facilitates movement of the paddles, promotes PTFE degradation which requires oxygen and aids the cooling process provided by the primary cooling water jacket. The cooling water must be heated prior to entering the cooling water jacket to avoid condensation.
The product resulting from the apparatus and methods described in U.S. Pat. Nos. 4,777,192 and 4,748,005 is said to be of poor quality and the powder particles treated with radiation in the ribbon-blender type apparatus are said to have widely varied sizes. The process is criticized as not cost effective in that particles mechanically comminuted to smaller sizes need less radiation and larger particles do not receive sufficient oxygen during treatment thus affecting the core. The process described in these patents is said to be basically, a batch process and requires large energy expenditures.
It has now been unexpectedly discovered that the molecular weight of polymeric material, such as PTFE, can be effectively degraded on a batch or continuous basis by movement of the material on a specially adapted vibratory table. The vibratory table is adapted to include a means for generating an electron beam, cooling water systems, conduits and means for delivering PTFE to the table so that the material, such as PTFE, moves under the electron beam at an even depth (e.g. consistent thickness) as a result of the combination of the delivery means and vibratory action of the table. The cooling water system is a dual loop system which permits effective heat transfer at the vibratory table within the design requirements of the table. Furthermore, the cooling water is at an initial temperature of about 55xc2x0 F. to 65xc2x0 F. which limits condensation, this consequently limits the potential for generation of hydrofluoric acid thus achieving a practical system using a cooling water jacket while alleviating the drawbacks encountered when water jackets are used with conventional and other known methods and apparatus. Also, dry air can be introduced at the trough of the vibratory table which inhibits condensation.
The apparatus and method comprising a vibratory table accomplishes several fundamental improvements over methods and apparatus known in the art. The apparatus and method transports an even layer of material on a continuous basis thus greatly reducing the depth-dose characteristics and other radiation processing inefficiencies associated with known methods, including the tray method; provides for an enclosed environment at the point of treatment which among other benefits protects against gas evolution thereby addressing hydrogen fluoride gas issues associated with known apparatus and methods and provides continuous and effective cooling which minimizes hydrofluoric acid production. When adapted with a pneumatic transport system to deliver material to and from the vibratory table area, the apparatus and process provides the added benefit of reducing the particle size of the material to between about 30 microns and about 3,000 microns.
Movement of material through vibratory action, in general, is known, however the specific concerns pertinent to radiation processing, that is movement of an even layer of material along a vibratory table with effective heat transfer, are not addressed. For example, U.S. Pat. No. 4,313,535 to Carmichael concerns an excited frame, vibratory conveying apparatus. U.S. Pat. No. 4,844,236 to Kraus concerns a vibratory conveying apparatus with a xe2x80x9cv-shapedxe2x80x9d trough useful for conveying material up an incline. A vibratory conveyor with a flexible trough attached to a spring bed which vibrates causing the flexible bed to flex and change its static shape thus thrusting the material upwards, which is particularly useful for sticky or adhesive materials, is discussed in U.S. Pat. No. 4,482,046 to Kraus. The apparatus in U.S. Pat. No. 4,482,046 can be adapted with a cooling system comprising a cooling jacket beneath the bed of the trough that uses air or water as the cooling medium, and the bed may be perforated with small holes to permit air or water flow and facilitate heat transfer. None of these patents discuss the requirements necessary for a vibratory table to deliver an even layer of material, such as PTFE, under an electron beam, and these patents do not address the criteria necessary for radiation treatment, in particular the removal of the significant heat generated at the vibratory table during radiation processing or cooling systems capable of performing in accordance with the requirements necessary for radiation treatment of fluoropolymers.
It has been unexpectedly discovered that the molecular weight of PTFE can be degraded efficiently and minimizing the problems associated with treatment of PTFE on a tray, and other processes known in the art. The apparatus and method used for the radiation treatment of material, such as PTFE, comprises 1) a vibratory table adapted for radiation treatment in an enclosed environment, 2) a dual loop cooling water system which effectively removes heat from the vibratory table, 3) a means for generating an electron beam, 4) a device that delivers material to the vibratory table which, along with the vibratory action of the table, allows for movement of an even layer of material beneath the electron beam, 5) cooling water systems for other apparatuses used in the method and 6) means to transport the material to the apparatus and to transport treated material from the apparatus to a staging and/or packaging area. The system is preferably operated on a continuous basis.
All percentages set forth herein are, unless otherwise noted, weight by weight percent.
A method and apparatus are described herein which provides for radiation treatment of material, preferably PTFE, which treatment can be performed on a continuous or batch basis. The material is transported under an electron beam on a vibratory table. The electron beam scans an aperture in a cover on the vibratory table which has a window below which the electron beam breaks the molecular chain of the polymer thereby reducing its molecular weight. The use of a vibratory table, combined with means for delivering material to the table, allows for an even layer of material to be moved under the beam, that is the material retains a consistent thickness as it moves along the vibratory table and under the electron beam. Thus, the radiation dosage can be adjusted to conform to the depth of material on the vibratory table, thereby alleviating the problems and inefficiencies associated with conventional methods and apparatus caused by the depth-dose characteristic, overscan and the need for a nominal dose. The method and apparatus described herein are superior to the tray methods and other known methods of treating polymeric material such as PTFE because the material is moved under the beam on a continuous basis without any gaps and, as such, efficiency losses associated with product gaps during treatment are alleviated. The method and apparatus using the vibratory table results in a degraded polymer product that is superior to those obtained through conventional and other known methods and the degraded polymer product treated on the vibratory table does not have the inconsistent properties and unpredictable product quality associated with degraded polymeric material subjected to radiation treatment by known methods and apparatus. The material processed on the vibratory table will also have better processability than material made with known processes.
The apparatus used in conjunction with the method comprises a system for delivering the material to the apparatus for treatment and a system for removing the material from the apparatus. These systems may be preferably pneumatic, however, any system capable of moving polymers is acceptable. The system for moving material to and from the processing area may be equipped with a separator or classifier that removes from the conveying system larger pieces created from fusion under the electron beam. Such a separator or classifier may also be used to automatically recirculate oversized particles back into the system. In one embodiment, two delivery lines may be co-joined which allows for mixing of polymers or other material in a delivery line so that a blend of polymers are delivered to the vibratory table. This embodiment is particularly useful when a reaction between polymers is desired which requires the energy of the electron beam. As such, the method and apparatus can be used for reactive coating to form co-polymers, including co-polymers comprising PTFE.
The apparatus is equipped with a unique system for delivering material to the vibratory table. PTFE, or other polymers, are placed on the vibratory table through a distribution chute specifically designed to provide even spreading of polymer on the trough of the vibratory table and compensate for pulse release of material from a plug flow rotary valve used to remove the material from a delivery line. In the embodiment involving pneumatic transport of PTFE, for example, PTFE is released from the pneumatic transport system to a plug flow rotary valve which moves the PTFE to the distribution chute. Thus, in this embodiment, the chute both evenly distributes the PTFE on the trough and compensates for the action of the plug flow rotary valve.
The distribution chute, which is in the shape of a hollow cone bisected in half, has a top opening defined by a rim which is half circular in shape and hollow with a diameter preferably about six inches, a hollow open conical section with an open annular space extending downwardly from the top opening to an opening at the bottom of the conical section, a bottom opening, defined by a rim which is hollow and a half circle with a diameter exceeding the diameter of the top opening. The hollow cone shaped distribution chute may be covered. The distribution chute comprises an internal directing plate which can be adjusted to equalize material flow across the trough of the vibratory table. Preferably the diameter of the bottom opening is approximately the size of the width of the bed of the vibratory table. A recessed flap is attached to the end of the distribution chute and extends angularly downward towards the vibratory table. The recessed flap restricts flow thus facilitating even distribution on the trough. Material enters the distribution chute at the top opening and then moves in a downward direction expanding axially to the diameter of the open conical section as the material descends in the conical section. The material leaves the distribution chute through the bottom opening and recessed flap and is thereby placed onto the trough of vibratory table in an even layer, and the vibratory action of the table maintains the even layer (e.g. consistent thickness) as the material passes beneath the electron beam. The distribution chute comprises an internal directing plate which can be adjusted to equalize the flow of material across the trough of the vibratory table. The chute assists in distribution of material as well as provides a continuous flow to the trough of the vibratory table.
The material is subjected to treatment on a vibratory table which comprises a cover that is sealed with the table to allow for control of the environment where the radiation processing occurs. The cover also has an aperture to accommodate a window for the scanning electron beam which allows the energy of the electron beam to interact with the material on the table. The window, which when removably affixed to the cover becomes a component of the cover, functions, in part, to enclose the environment at the point of radiation treatment, such as at the trough of the vibratory table, while allowing the electron beam into the enclosed environment for treatment. Thus, the window may be made of any material that does not significantly resist the transmission of the electron beam. In an embodiment of the invention, the window comprises a titanium sheet and PVC material may be wrapped around the opposite ends of the titanium sheet, with, optionally, a metal or composite frame to provide structural support to the titanium sheet and allow for replacement or change-over of the window. The window may also comprise a plurality of titanium sheets stacked upon one another with two opposite ends with PVC material wrapped around opposite ends of the plurality of titanium sheets and the plurality of titanium sheets may reside within a metal or composite frame to give structural support and provide for ease of change-over or replacement of the window. The window may be equipped with a cooling system.
The electron beam provides energy for treatment of material, such as the degradation of the molecular weight of PTFE and/or reaction of PTFE with other polymers and chemicals. Through degradation the molecular weight of the material is reduced. The vibratory table is capable of flow rates up to about 5,000 pounds of material per hour, such as up to about 2,500 pounds of material per hour. When a pneumatic flow system is used to transport the material to and from the vibratory table, the pneumatic transport combined with the radiation processing which causes embrittlement of the polymer, allows for particle size reduction of the polymer, and the material can be reduced to a particle size of between about 30 microns and about 3,000 microns in single or multiple passes beneath the beam.
The vibratory table is adapted to accommodate a means for generating an electron beam. The electron beam can be generated by a particle accelerator which energizes and accelerates electrons which are then focused into a beam and directed by a scan horn to the window at an aperture in the cover of the vibratory table. A turbo vacuum pump creates a vacuum for the accelerator and electron beam. The beam moves through the window and into the path of the material as it moves on the bed of the vibratory table under the window thus causing the degradation of the molecular weight of the material. The radiation dosage is adjusted depending on the depth of material on the bed. For degradation of polymeric material, such as PTFE, the dosage can be about 2 megarads (xe2x80x9cMradsxe2x80x9d) to about 10 Mrads at a setting of about 1.5 Megavolts (xe2x80x9cMeVxe2x80x9d) with amperage settings of about 5 to about 40 milliamps. However, material depth on the bed can be adjusted depending on the dosage. The depth of material on the bed as the material passes under the electron beam is nearly constant. The material may be at a near even depth, that is a consistent thickness of between about xe2x85x9 inches to about xc2xd inches, depending on the bulk material density.
The extent of degradation of the molecular weight of the material is a function of the depth of material on the bed of the vibratory table (that is the thickness of the material), the dosage and the size and/or original molecular weight of the material. Thus, to achieve a desired molecular weight of the material, repetitive treatments may be necessary. Through experimentation, the depth of material, radiation dose and number of passes necessary to achieve a desired molecular weight can be determined and thus precise standards developed. When pneumatic transport system is used in conjunction with the radiation processing, the parameters such as depth of material, radiation dose and number of passes through the apparatus to achieve a desired particle size having material of desired molecular weight can be determined. Generally, particle sizes of about 100 microns to about 3,000 microns can be achieved with a single pass through the apparatus with pneumatic transport and from about 30 microns to about 3,000 microns with multiple passes. As such, without extensive product analysis or separation techniques, each time the product, such as PTFE, is treated, the parameters effecting molecular weight degradation can be determined and available for commercial use of the method and apparatus. When equipped with a pneumatic transport system, the parameters affecting particle size reduction may also be determined. Accordingly, the apparatus can be operated on a routine basis without the need for expensive and cumbersome analysis each time material is processed on the table to determine if additional processing is necessary.
A means to comminute the material after radiation processing may also be used to reduce the polymeric material to a desired particle size after radiation processing. For example, the polymeric material may be pulverized by one or more mills or micronized by one or more jet mills.
The material is placed on the trough of the vibratory table through the distribution chute which provides for even distribution of material on the bed. The vibratory table can be slightly inclined at an angle, and, if so, the material is deposited on the table at the inclined end of the table. The process, however, can be operated with the vibratory table in the horizontal position. The incline of the table is particularly helpful with respect to the cooling water system by providing that water will fill the cooling water jacket, and in particular channels within the jacket, while maintaining uninterrupted contact with the underside of the trough of the vibratory table. The material then flows in a forward direction along the trough and under the path of the electron beam. The table is capable of flow rates of up to about 5,000 pounds of material per hour, preferably up to about 2,500 pounds of material per hour. The trough of the vibratory table comprises a bottom and sides protruding at an upward angle from each side edge of the bottom of the trough and flanges that protrude from the sides at an angle of repose about horizontal with reference to the bottom of the trough. The upwardly protruding sides prevent PTFE from falling off the sides of the trough. The trough is preferably stainless steel made of a thin material, preferably no more than about 0.08 inches to about 0.37 inches thick, preferably about 0.125 inches thick. The upper side of the trough, that is the side which holds the material, is highly polished, up to about a 240 grit finish. The thinness of the trough facilitates heat transfer and the polished surface serves to both facilitate heat transfer and reduce friction between the material and trough to allow for smooth movement and maintain the desired even depth of the material on the table.
The electron beam is directed to the window, which is rectangular and is located above the table at approximately the mid-point of the trough, and scans the window. The window may be positioned in any manner at the midpoint of the cover, depending on the size of the scan horn of the particle accelerator, and may be located perpendicular to the direction of material flow or at an angle. The entire table is covered by a cover attached to the flanges and the window is placed into an opening, an aperture, in the cover and the window is attached to the cover. One or a plurality of gaskets, preferably two inch silicone gaskets, frame the material of the window. The window is secured to the cover by a variety of means, including gaskets, frames, welds or any other means, or combination of means, to secure the articles together while maintaining a seal for the enclosed environment. A full face gasket may be located over the silicone gasket to seal the window against a stainless steel plate which is then tightened down to secure the window and plate against the table cover, thus securing the window to the cover.
The material is treated in an enclosed environment. This is accomplished by the cover and attached components which are attached to the vibratory table. The enclosed environment allows for treatment in an infinite number of atmospheric conditions within the enclosed environment at the vibratory table. For example, the treatment can occur in an oxygen rich environment, inert environment or nitrogen rich environment depending on needs and requirements. The enclosed environment also allows for chemical treatment and reaction of the material. The apparatus can easily be adapted to provide for chemical reactions of the polymeric material at the table, such as PTFE, and substances permitted into the atmosphere in the enclosed environment. Generally, oxygen facilitates degradation of PTFE and, as such, a preferred embodiment of the invention is to charge the enclosed environment at the vibratory table with dry air which both facilitates PTFE degradation and inhibits condensation.
The cover, and enclosed environment, also alleviates the problems associated with the generation of hydrogen fluoride gas associated with conventional radiation treatment methods for fluoropolymers, such as PTFE. Because the environment at the treatment area is enclosed, hydrogen fluoride gas cannot migrate from the table thus limiting effects on equipment and limiting worker safety concerns. Also, the vibratory table is located within a vault further limiting the potential for hydrogen fluoride gas migration. The cooling water system, by use of water which enters the system at about 55xc2x0 F. to 65xc2x0 F., also inhibits condensation. Furthermore, the controlled environment allows the operators to create conditions which inhibit hydrogen fluoride gas production or condensation within the enclosed environment at the vibratory table. As discussed above, dry air can be used which inhibits condensation. Thus, the enclosed environment at the vibratory table inhibits or alleviates the problems of equipment corrosion, worker safety concerns and environmental control issues arising from hydrogen fluoride generation, and hydrofluoric acid production, that are generally associated with radiation treatment systems for fluoropolymers known in the art, including but not limited to tray methods.
The vibratory table is equipped with a jacket which surrounds the undersides of the bottom and sides of the trough. Cooling water, or other heat transfer fluid media, is a circulated in a continuous closed loop to channels located and in contact with the underside of the trough, including the upwardly protruding sides. The cooling water or other heat transfer media, flows opposite to the direction of material on the vibratory table, which both facilitates heat transfer and allows for gradual water heating which further inhibits the formation of condensation and formation of hydrofluoric acid when PTFE or other fluorinated polymers are treated. The water, or other heat transfer media, is circulated at a flow rate of about 15 to about 25 gallons per minute (xe2x80x9cgpmxe2x80x9d) at pressure about 5 psi to about 7 psi. Heat transferred to the water from the table is removed from the water in the closed loop through a heat exchanger. The heat exchanger uses a heat transfer medium, preferably a mixture of ethylene glycol and water to remove heat from the cooling water. The heat transfer medium, such as ethylene glycol and water, is circulated between the heat exchanger and a chiller in a separate loop, independent of the closed loop for the vibratory table, thus the cooling water system for the vibratory table is a dual loop system. Other pieces of equipment used in the method also have cooling systems, some of which may also use an ethylene glycol and water mixture as a heat transfer medium which can be cooled in the same chiller used to remove heat from the heat transfer medium used for the dual loop system for the vibratory table.
As will be readily understood by one skilled in the art, control equipment appurtenant to the apparatus is necessary for the system to operate. These include, but are not limited to; piping, circulating pumps, valves, blowers, coolers, power sources and filters. Also, the system is controlled from a station which allows the operators to control flow variables with the aid of microprocessors.
In general, material, such as PTFE, is moved to a flake unloader-separator where it is mixed with air, and the material is then fed under gravity to a feed hopper. The material is then transported to a plug flow rotary valve which then allows the material to flow under gravity into the distribution chute. The distribution chute evenly distributes the material onto the trough of the vibratory table and the material moves in the trough under an electron beam. After processing, the material moves to the end of the trough opposite to the distribution chute where it drops under gravity into a finished product hopper. The material exits the finished product hopper through a rotary valve and is then transported from the area of the vibratory table by a second transport system which may be a pneumatic transport system. Finished product may be moved to a packaging crate or returned to the product hopper for further treatment. The process can be adapted to include a separator or classifier that removes from the conveying system larger pieces created from fusion under the electron beam and can also be used to automatically recirculate over-sized particles back into the apparatus.